This application claims the priority of 197 55 813.5, filed Dec. 16, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process for operating a system for the water vapor reforming of a hydrocarbon and to a reforming system which can be operated according to this process. The invention particularly relates to a process for operating a mobile system for the water vapor reforming of methanol in a fuel-cell-operated motor vehicle for providing the hydrogen required for the fuel cells and to a system which can be operated in this manner, as well as to an operating process of a corresponding fuel cell system. For reasons of simplicity, the term "hydrocarbon", in addition to the actual hydrocarbons, will also include their derivatives, such as methanol.
In water vapor process for reforming hydrocarbons, a hydrogen separating stage having a selectively hydrogen permeable membrane separates the hydrogen generated by the reforming reaction in the warmed-up operation from the other constituents of the formed reformate gas. In addition to alternative methods, such as the CO conversion to carbon dioxide by way of a CO oxidation or the so-called CO shift reaction, this represents a method for obtaining a product gas which essentially consists of hydrogen, in which the CO concentration does not exceed a defined low threshold value. This is important, for example, when the product gas is used as the anode gas of a fuel cell system because the carbon monoxide acts as a catalyst poison. The hydrogen separating stage can be connected as a separate unit to the reforming reactor or can be integrated in the reforming reactor.
As known, the water vapor reforming reaction for reforming a hydrocarbon or hydrocarbon derivative, such as methanol, takes place endothermally and at a reaction temperature which is higher than the room temperature. During a cold start of such a system, the water vapor reforming reaction does not immediately provide hydrogen. The system components must first be brought to a corresponding heated operating temperature. However, particularly when the systems are used in motor vehicles, it is desirable to have driving power by the fuel cells available as soon as possible after the triggering of a starting operation of the vehicle and thus also of the reforming system. This requires, in turn, that the reforming system be capable of providing hydrogen as quickly as possible, at as low a cost as possible. Various special measures for the cold start of reforming systems have been suggested for this purpose.
It is known from French Patent Documents FR 1.417.757 and FR 1.417.758 to introduce during a cold start of a water vapor reforming system for methanol first a mixture of methanol and an oxidant into the reforming reactor in order to carry out a corresponding combustion reaction and thus heat the reactor. The oxidant feed is then terminated. The methanol/water vapor mixture to be reformed is fed and the water vapor reforming reaction is started. In the case of the system of French Patent Document FR 1.417.757, a heating space is in thermal contact with the reforming reaction space. In the heating space, residual gas from the reaction space which is not diffused through a separating membrane is non-catalytically burned with oxygen. An analogous measure is described in Japanese Published Patent Application JP 4-321502 (A).
German Patent Document DE 44 23 587 C2, discloses obtaining hydrogen optionally by means of an exothermal partial oxidation and/or an endothermal water vapor reforming of methanol in a reforming reactor filled with a suitable catalyst material, such as a Cu/ZnO material, depending on the control of the feeding of the individual reaction partners into the reactor and the temperature existing there. When the process is carried out appropriately, the two reactions will take place in parallel, in which case an autothermal reaction course can be set.
It is also known to use the anode-side exhaust gas of a fuel cell system fed with hydrogen by a reforming system directly or after an intermediate storage for heating the reforming reactor. See, for example, Japanese Published Patent Applications JP 4-338101 (A), JP 4-160003 (A) and JP 2-160602 (A). Japanese Patent Document JP 4-338101 (A), is used especially for starting the system, and Japanese Patent Documents JP 4-160003 (A) and JP 2-160602 (A), is additionally used for the reforming reaction in the reforming reaction space while heat is additionally generated by a partial oxidation reaction.
In the fuel cell system described in U.S. Pat. Document U.S. Pat. No. 5,248,566, the fuel cells are fed by hydrogen on the anode side. This hydrogen is generated by a partially oxidizing reforming reactor, the anode exhaust gas of the fuel cells is burned in an afterburner while feeding air. The resulting generated heat is used for heating the interior of a motor vehicle which is equipped with the fuel cell system.
Special cold starting measures were also suggested for systems for the water vapor reforming of a hydrocarbon without the use of a hydrogen separating stage. U.S. Pat. Documents U.S. Pat. No. 4,820,594 and U.S. Pat. No. 5,110,559 describe systems for water vapor reforming hydrocarbon in which a burner is integrated in the reforming reactor. The reforming reactor is in thermal contact with the reaction space of the reactor by way of a heat-conducting partition. During cold start, a combustible mixture is burned in this burner at an open flame. In U.S. Patent Document U.S. Pat. No. 5,110,559 the flame originates from the reforming reactor itself, the combustible hydrocarbon to be reformed being fed to the reaction space during the cold start. The hot combustion exhaust gases of the burner integrated in the reactor are guided into a CO shift converter connected in order to heat it and in this manner bring the system to the operating temperature more quickly.
A problem occurs, however, when the process of partial oxidation of the hydrocarbon, i.e., the POX process, is used in connection with a selectively hydrogen-separating member. A sufficiently high operating pressure, typically above 10 bar at the membrane, is required for achieving a sufficient hydrogen diffusion capacity. Simultaneously the POX process requires an oxygen-containing gas, such as air, which must therefore be compressed to the membrane operating pressure, which leads to correspondingly higher costs.
The present invention addresses the technical problem described above by providing a process and a system for the water vapor reforming wherein the system components reach their operating temperature as quickly as possible during a cold start at relatively low cost. Hydrogen can be provided correspondingly rapidly and can optionally be used in fuel cells. The present invention is also directed to a fuel cell system operating process of the initially mentioned type.
By means of the process according to the invention, during a cold start, the reforming system can be brought comparatively rapidly to its normal warmed-up operating condition without major expenditures. A heating operation is carried out wherein first the reforming reactor, which can be designed for POX operation as well as for water vapor reforming is operated at a relatively low pressure in a POX operation. The exothermal POX process generates heat, which, depending on the system construction, is transported via a direct solid-state heat conduction and/or by product gas generated during the partial oxidation as a heat carrier medium into the hydrogen separating stage and heats the membrane there. The product gas emerging from the reactor which, because of partial oxidation of the hydrocarbon, already contains hydrogen, is then transmitted from the hydrogen separating stage to the catalytic burner device and is catalytically burned there. Since the burner device is in thermal contact at least with an evaporator and the reforming reactor, these system components are also rapidly heated. Although supplementary heating measures may be provided, such as electric heating of the evaporator, the reforming reactor and/or the hydrogen separating stage, or directly feeding a catalytically combustible mixture into the catalytic burner device, this is not absolutely necessary.
When at least the reforming reactor has reached a normal, heated operating temperature, used in normal operation during the water vapor reforming, a second operating phase of the heating operation begins. The POX process is stopped and the system is run up to the normal operating pressure of, for example, between 10 bar and 40 bar required for the hydrogen-separating membrane. Simultaneously, water and the hydrocarbon to be reformed are fed into the evaporator and their evaporation is started in order to feed the forming mixture to the reactor and to reform the hydrocarbon. With the increased operating pressure and the continuously rising membrane temperature, the membrane becomes increasingly permeable for hydrogen. When used in a fuel-cell-operated motor vehicle, the system will already be supplied with hydrogen by this point in time that, by means of this hydrogen, unlimited driving operation is possible by means of the power of the fuel cell system.
According to one embodiment of the present invention, the membrane is heated not only by product gas guided through the hydrogen separating stage but also by a catalytic combustion activated in the hydrogen separating stage itself or in a part of the catalytic burner device which is in thermal contact with the hydrogen separating stage. As a result, the membrane will reach its normal operating temperature even faster. This process is particularly suitable for operating the system using a intermediate supply of air and hydrogen.
In another embodiment of the present invention, electrically generated heat is fed to the reforming reactor for a short time at the start of the first operating phase. This promotes fast starting of the POX operation of the reactor.
In another embodiment of the present invention, in the first operating phase of the heating operation, the combustion exhaust gas of the catalytic burner device is guided through the evaporator and/or the membrane so that these components are heated even faster and reach their normal operating temperature. The additional membrane heating can be implemented particularly by means of heating ducts.
In another embodiment of the present invention, the hydrocarbon which is subsequently reformed, is fed during the heating operation directly into the catalytic burner device for the purpose of a catalytic combustion. As a result, the catalytic burner device is capable of heating the reforming reactor and/or the evaporator and/or the hydrogen separating stage during time periods where no corresponding hydrogen from the POX operation of the reforming reactor has yet been fed to the catalytic burner device. This occurs, for example, because a sufficient amount of hydrogen has not yet been generated or because considerable amounts of hydrogen are already diffusing through the membrane and are used for different purposes.
In another embodiment of the present invention, at least one intermediate feeding conduit or fuel feeding pipe is provided so that water can be metered and fed into the reforming reactor. This fuel may be similarly fed into the hydrogen separating stage and/or into the catalytic burner device in order to operate as a heat transport medium and simultaneously avoid an overheating.
In a further embodiment of the present invention, the process may be used for a system with a multi-part catalytic burner device which has at least one burner part for the reforming reactor and a burner part for the evaporator. According to this process, oxygen-containing gas can be fed individually to different burner parts so that the chemical combustion energy of directly fed fuel or of the product gas coming from the reforming reactor and fed into the catalytic burner device can be distributed in a targeted manner to the individual burner parts.
In a further embodiment of the invention during the heating operation, the evaporator end the reforming reactor are heated to a temperature which is above the normal operating temperature in order to bring the system as a whole faster to normal operating conditions, and particularly be able to more rapidly heat the hydrogen separating stage.
In another embodiment of the invention, fuel cell system operating process is particularly suitable for use in fuel-cell-operated motor vehicles. In this case, the hydrogen generated by the reforming process is used as fuel for the fuel cells. The hot combustion exhaust gas of the catalytic burner device is used for heating a cooling circulation system of the fuel cell system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.